1. Field of the Invention
The present invention relates to control of the charge accumulation time or charge accumulation area of a focus detection device using a charge accumulation type image sensor.
2. Related Background Art
A focus detection device, which forms an object image on a charge accumulation type image sensor by object light transmitted through a photographing lens, and detects a focus state of the photographing lens on the basis of the object image signal obtained from the image sensor, is known.
In a focus detection device of this type, an object image formed on a focus detection area set on a photographing frame is photoelectrically converted by an image sensor.
When a charge accumulation type image sensor is used, the charge accumulation time must be controlled, so that the obtained object image signal has a level suitable for a focus detection calculation regardless of the luminance of an object. As a method of controlling the charge accumulation time, for example, a method of controlling the accumulation time, so that the peak value of the object image signal has a level value suitable for the focus detection calculation, is known. No problem is posed when focus detection is executed by this control method in a normal photographing situation in which a luminance difference between a main object and a background portion is small.
However, in a situation requiring a flash photographing operation, high-luminance objects such as street lamps, neon signs, and the like are often present as a background portion of a relatively dark main object.
In the conventional focus detection device, when both a main object and a very high-luminance background portion are present within a focus detection area, the charge accumulation time of the charge accumulation type image sensor is controlled, so that an object image signal has a level suitable for the high-luminance background portion. Therefore, an object image signal of the background portion has proper contrast to enable focus detection of the background portion. However, an object image signal of the main object portion becomes a low-level output, and proper contrast cannot be obtained, thus disturbing focus detection.
For this reason, since the focus state for the background portion is detected, and the lens is driven according to the detection result, the background portion is focused, and the main object to be originally focused suffers from an out-of-focus state.
Such a state will be described in detail below.
For example, a case will be examined below wherein a person is located at the center of the frame, and houses are located as a background in a dark state requiring a flash photographing operation, as shown in FIG. 6A. When a focus detection area 51 is set, as shown in FIG. 6B, the accumulation time is controlled, so that an object image signal corresponding to light from window portions of the houses has a predetermined level, as shown in FIG. 7A. As a result, the focus state for light from the windows is detected, and since a central signal portion has a low level, as shown in FIG. 7B, the conventional focus detection device cannot detect the focus state for the person.
The same problem is posed in a photographing operation executed in a rear light condition wherein a brighter background is present behind a main object.